Geochronology and Geological Implication in Two Episodes of Meso-Neoproterozoic Magmatism in the Southwestern Yangtze Block
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摘要: 扬子陆块西南缘发育一系列中-新元古代岩浆岩,对认识扬子陆块构造演化具有重要意义.对会理地区天宝山组流纹岩和盐边地区辉长岩进行SHRIMP锆石U-Pb年代学、地球化学研究.天宝山组流纹岩时代为1 011.9±8.9 Ma,辉长岩时代为910.6±4.7 Ma.天宝山组流纹岩具有高硅、高FeOt/MgO、高钾等特征;稀土含量(∑REE=292×10-6~401×10-6)较高,表现出轻稀土富集重稀土弱亏损的特征[(La/Yb)N=1.77~6.74],Eu负异常明显(δEu=0.43~0.56),与A型花岗岩相似;天宝山组流纹岩来自古老的地壳物质的部分熔融,形成于大陆裂谷环境.盐边群辉长岩稀土含量(∑REE=54×10-6~98×10-6)较低,轻重稀土分异较弱[(La/Yb)N=1.46~4.72],Eu具有轻微的异常(δEu=0.81~1.31);岩石具有明显的Nb-Ta、Ti负异常,无Zr-Hf正异常;地球化学数据显示辉长岩来自被俯冲板片释放的流体/熔体交代的地幔楔部分熔融,形成于岛弧环境.两期岩浆活动指示了扬子西南缘1 000~910 Ma之间构造动力学背景发生了转变,由伸展背景转变为挤压背景.
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关键词:
- 扬子西南缘 /
- 中-新元古代 /
- SHRIMP锆石U-Pb年龄 /
- 地球化学 /
- 大地构造背景
Abstract: Late Mesoproterozoic to Early Neoproterozoic igneous rocks occur in the southwestern Yangtze block, which had a great bearing on the evolution history of the Yangtze block during the late Mesoproterozoic to early Neoproterozoic. This study reports SHRIMP zircon U-Pb ages and geochemistry data for gabbros that intruded in the Yanbian Group, and that of rhyolites from the upper Tianbaoshan Formation of the Huili Group in the southwestern Yangtze block.The rhyolites were dated at 1 011.9±8.9 Ma and the gabbros were formed at 910.6±4.7 Ma. The rhyolites in the Tianbaoshan Formation were characterized by high SiO2 and K2O contents and high FeOt/MgO ratio. The contents of rare earth elements of rhyolites are high(∑REE=292×10-6-401×10-6), and characterized by LREE-enriched and HREE-depleted patterns[(La/Yb)N=1.77-6.74] with typical depletion of Eu(δEu=0.43-0.56), consistent with the geochemical characteristics of A-type granites. The geochemistry indicates that the rhyolites were derived from the partial melting of previous crust and formed in a continental rift setting. The gabbro shave low rare earth elements(∑REE=54×10-6-98×10-6) and characterized by slightly LREE-enriched and HREE-depleted patterns with unconspicuous Eu anomaly[(La/Yb)N=1.46-4.72, δEu=0.81-1.31] and the trace element patterns with typical depletion of Nb-Ta and Ti but no enrichment of Zr-Hf. The gabbros were derived from the subduction-modified lithospheric mantle wedge and formed in an arc setting. In view of the two episodes of magmatism in the study region, we propose that the tectonic properties changed from a continental rift setting to a compression setting in the southwestern Yangtze Block at 1 000-910 Ma. -
0. 引言
随着对Rodinia超大陆汇聚与裂解研究的深入,华南地区在超大陆的重建中扮演着越来越重要的角色(Greentree et al., 2006; Zhou et al., 2006a, 2006b; Li et al., 2008; Li et al., 2009; Hu et al., 2017; Zhao et al., 2017; Li and Zhao, 2018; 张克信等, 2018; 唐增才等, 2018; Wang et al., 2019a, 2019b; Zhu et al., 2019a, 2019b).传统观点认为华南由扬子陆块和华夏陆块沿江南造山带拼贴而成,但是两大陆块的拼贴时限长期存在争议.一部分学者认为扬子与华夏陆块拼贴时限为1 000~900 Ma,江南/四堡造山带属于全球格林维尔造山运动的一部分(Greentree et al., 2006; Li et al., 2006, 2009; Li et al., 2008).根据获得的最新的年代学和地球化学数据,另一部分学者提出两大陆块最终拼贴时间为820 Ma,甚至更晚,江南/四堡造山带远滞后于全球格林维尔造山运动(Zhou et al., 2006a, 2006b; Wang et al., 2014; Chen et al., 2018).因此,查明华南地区是否存在格林维尔期造山运动对认识华南中-新元古代构造演化具有重要意义.
扬子陆块西南缘是华南中元古代地层出露最完整的地区,是认识板块构造演化的窗口.近年来随着测年技术的不断更新,越来越多的中元古代晚期-新元古代早期的岩浆活动被陆续报道(Greentree et al., 2006; Li et al., 2006; Zhou et al., 2006a, 2006b; Sun et al., 2008a, 2008b, 2009; 关俊雷等, 2011; Zhao et al., 2012; Zhu et al., 2016; 耿元生等, 2017; Zhao et al., 2017, 2018; Li and Zhao, 2018; Zhu et al., 2019a, 2019b).会理群天宝山组火山岩锆石U–Pb年龄为1 036~1 021 Ma(尹福光等, 2012; Zhu et al., 2016; 耿元生等, 2017; Chen et al., 2018);侵入盐边群的岩体时代为860~806 Ma(Li et al., 2006; Zhou et al., 2006a, 2006b; Sun et al., 2008a);昆阳群黑山头组凝灰岩SHRIMP锆石U-Pb年龄为1 032 Ma(张传恒等, 2007);苴林群变质玄武岩锆石U-Pb年龄为1 050 Ma(Chen et al., 2014);Wang et al.(2019a)报道了会东地区1 040 Ma的A1和A2型花岗岩.以上成果对扬子陆块的构造演化历史及超大陆的重建提供了丰富的资料.但是上述岩浆岩的成因及构造背景的认识仍然没有得到统一,加之缺少系统的同位素地球化学研究,致使扬子陆块西南缘构造演化机制长期存在争议.
本文以扬子西南缘会理群天宝山组流纹岩及侵入盐边群的辉长岩为研究对象,展开年代学及地球化学分析,以期补充完善扬子西南缘中-新元古代岩浆活动资料,进一步理解扬子西南缘中-新元古代的区域大地构造演化过程.
1. 地质背景
1.1 区域地质概况
扬子陆块西缘与松潘-甘孜地块以龙门山断裂带为边界,北缘与华北克拉通以秦岭-大别造山带为界,东南缘以江南造山带与华夏陆块相邻(图 1a).扬子陆块前寒武纪基底主要以新元古代(860~740 Ma)岩浆岩和变质沉积岩为主,同时出露少量古-中元古代和太古代地层.太古代基底地层主要为扬子北缘的崆岭杂岩和鱼洞子杂岩.崆岭TTG杂岩时代为3.3~2.6 Ga(Guo et al., 2014; Han et al., 2017; Zhao et al., 2018; Lu et al., 2019),后期被1.85 Ga的A型花岗岩-基性岩脉及860~790 Ma黄陵花岗杂岩所侵入(Peng et al., 2009; Han et al., 2019);鱼洞子杂岩主要包括变沉积岩和TTG组合,时代为2.7 Ga(Hui et al., 2017; Zhou et al., 2018).古-中元古代火山-沉积岩地层序列主要分布在扬子陆块西缘、西南缘及北缘(Li and Zhao, 2018).新元古代火山-沉积岩地层及各类侵入岩主要围绕扬子陆块周缘分布(Zhao et al., 2018; Zhu et al., 2019a, 2019b).
图 1 (a) 华南板块构造格架简图;(b)扬子西南缘川滇地区元古代地层分布图(据耿元生等, 2017修改)Fig. 1. (a) Simplified tectonic framework of the South China block; (b)geological map of the distribution of Proterozoic strata in Yunnan–Sichuan provinces(modified from Geng et al., 2017)扬子陆块西南缘,纵贯川滇两省,研究区内自北向南广泛发育一系列前寒武纪高级变质岩系及低级变质沉积岩(Zhu et al., 2016; Chen et al., 2018),受多期次构造影响,各构造单元内的地层均呈断块产出.高级变质岩主要以四川省内的康定杂岩为代表,早期研究认为该高级变质岩作为扬子西南缘的结晶基底,时代为古元古代-太古代(吴根耀, 2006).但是近期年代学研究表明康定杂岩等高级变质岩时代为824~764 Ma (耿元生等, 2008; Zhao et al., 2018),主要形成于新元古代.低级变质岩系主要包括中元古代早期-新元古代早期的基底地层(本文中元古代底界以1 800 Ma为界).中元古代早期地层主要包括河口群、大红山群和东川群及相当地层.东川群主要由板岩、砂质白云岩、粉砂质硅质板岩及碳酸盐岩组成,下部因民组凝灰岩时代为1 742±13 Ma (Zhao et al., 2010),中部黑山组凝灰岩年龄为1 503±17 Ma、1 504±5 Ma (耿元生等, 2017),属于中元古代早期.大红山群和河口群主要为变质火山-沉积岩系,大红山群主要以变质沉积岩和变质火山岩为主,火山岩测年结果显示大红山群地层时代为1 722~1 675 Ma(Zhao et al., 2012),属于中元古代早期.河口群主要由石英钠长岩(细碧角斑岩)与片岩、大理岩等组成,下部大营山组火山岩定年结果为1 722±25 Ma、1 705±6 Ma(Chen et al., 2013; 耿元生等, 2017).侵入河口群的基性岩脉年龄为1 657±21 Ma(Chen et al., 2013)和1 710±8 Ma(关俊雷等, 2011),显示河口群形成于中元古代早期.中元古代早期地层序列为陆内断陷盆地沉积,是Columbia超大陆裂解在扬子西南缘的响应(关俊雷等, 2011; Chen et al., 2013).中元古代晚期-新元古代早期地层包括会理群、昆阳群和盐边群及相当地层.会理群下部岩性为变质碎屑岩夹碳酸盐岩,上部为变质火山-沉积岩组合,上部天宝山组火山岩年龄为1 028~1 021 Ma(Chen et al., 2014, 2018; Zhu et al., 2016),属中元古代晚期;昆阳群以板岩、变质石英砂岩、粉砂岩、碳酸盐岩等为主,夹火山岩,Greentree et al.(2006)在昆阳群下部老吾山组凝灰岩获得1 142±16 Ma的SHRIMP锆石U-Pb年龄,中部黑山头组富良棚段凝灰岩年龄为1 032±9 Ma(张传恒等, 2007)、1 03l±12 Ma(尹福光等, 2012),属于中元古代晚期.盐边群下部荒田组火山岩年龄为841±10 Ma(耿元生等, 2008)和825±10 Ma(张传恒, 2007);Sun et al.(2008b)于盐边群碎屑锆石获得870 Ma的最大沉积年龄,侵入盐边群的岩体时代为806 Ma、812 Ma和858 Ma(Zhou et al., 2006a; Sun et al., 2008a),说明盐边群时代为新元古代早期.研究区低级变质岩系后期被大量860~750 Ma的花岗岩、闪长岩和辉长岩等侵入(Zhou et al., 2006a, 2006b; Chen et al., 2014).
1.2 会理群和盐边群
会理群主要出露在四川省会理、会东地区(图 1).自下而上分为力马河组、凤山营组和天宝山组(图 2) (四川省地质矿产局, 1991).力马河组厚约4 290 m,下部为深灰色硅质千枚岩、砂质千枚岩夹石英岩、变石英砂岩;中部为石英千枚岩、硅质千枚岩互层;上部为灰白、深灰色石英岩、变石英砂岩夹黄褐-灰黑色变粉砂岩、碳质千枚岩.凤山营组厚约2 700 m,为灰-深灰色薄层状泥质白云岩、泥质灰岩或泥灰岩的互层,夹少许薄-中层状石灰岩、白云质灰岩、白云岩,近顶部见钙质板岩,白云岩中偶含砂质,普遍具韵律层理、交错层理、底冲刷面、同生角砾等原生沉积构造,与下伏力马河组呈整合接触.天宝山组厚约1 000 m,下部为杂绿、杂灰绿色板状、绿泥绢云千枚岩夹薄-中层状变泥质白云质粉-细砂岩,向上夹杂黄绿色变英安质晶屑凝灰岩;上部为杂灰绿色变英安质晶屑凝灰岩、变英安质沉凝灰岩、流纹质凝灰岩、流纹斑岩、火山角砾凝灰岩夹浅绿灰色英安质绢云千枚岩、粉砂质千枚岩、变凝灰质粉砂岩(四川省地质矿产局, 1991).
盐边群出露于四川省攀枝花市盐边地区(图 1),自下而上被划分为荒田组、渔门组、小坪组和乍古组(图 3).荒田组以变玄武岩为特征,总体为灰绿色变玄武岩、变玄武质角砾岩夹粉砂质板岩、硅质板岩、泥硅质岩,向上变玄武岩减少,渐变为变安山质玄武岩、变安山岩,变玄武岩中发育杏仁状、枕状构造,未见底,厚度大于1 800 m.渔门组下部深灰色碳硅质板岩、绢云板岩,夹粉砂质板岩;上部为灰黑色碳质绢云板岩,夹中层状砂质板岩,板岩具条带状构造,厚度约1 700 m(四川省地质矿产局, 1991).小坪组总体为一套变质碎屑岩,由绢云板岩、砂质板岩、炭质绢云板岩夹变质砂岩组成,底部为厚层状变质凝灰质细砾岩和变质砂岩,见花岗岩脉和辉长岩脉侵入其中.厚度约2 200 m(四川省地质矿产局, 1991).乍古组主要为绢云板岩、粉砂岩和板岩,底部以变质凝灰质砾岩,下部夹炭质板岩、变质细砂岩和粉砂岩,上部夹互层状白云质灰岩与白云质板岩(四川省地质矿产局, 1991).
2. 样品及测试方法
2.1 样品特征
锆石测年样品20181207-7采自雅砻江畔侵入到盐边群的辉长岩,风化破碎较明显.辉长岩由斜长石、角闪石、黑云母组成.斜长石呈他形粒状,定向分布,具高岭土化、绢云母化.角闪石显蓝绿色,柱粒状,定向分布.黑云母呈鳞片状,定向分布.岩内见少量绿泥石沿裂纹分布,副矿物主要为不透明矿物、磷灰石等(图 4).样品20181206-1采自天宝山组变流纹岩.岩石由长石、石英、绢云母、黑云母、绿泥石组成,长石和石英呈他形粒状,定向分布.绢云母、黑云母、绿泥石呈隐微鳞片状,长轴多沿同一方向定向排列,具面状、条带状消光,黑云母显褐绿色.副矿物主要为不透明矿物、磷灰石等(图 4).
图 4 (a) 天宝山组流纹岩; (b)侵入盐边群的辉长岩; (c)会理群天宝山组流纹岩镜下特征; (d)侵入盐边群辉长岩镜下特征Qz.石英; Pl.斜长石; Ch.绿泥石; Hb.角闪石; Bt.黑云母Fig. 4. (a)Field photo of the rhyolites in the Huili Group; (b) field photos of gabbros the in the Yanbian Group; (c)microscopic photo of the rhyolites in the Huili Group; (d) microscopic photo of the gabbros in the Yanbian Group2.2 分析方法
锆石挑选工作在河北廊坊区调所实验室完成,阴极发光图像拍照在北京离子探针中心HITACHI S-3000N扫描电镜上完成,锆石U-Pb定年于中国地质科学院北京离子探针中心SHRIMP-Ⅱ上进行.仪器的质量分辨率为5 000(1%峰高),一次流O2–强度为4~6 nA.一次流束斑直径为23 μm,每个测点均扫描5次.元素间的分馏校正及U含量标定采用标准锆石TEM来进行;TEM具有U-Pb谐和年龄,206Pb/238U年龄为416.8±1.1 Ma,但U、Th及Pb含量不均一.采用Ludwig博士编写的Squid和Isoplot程序进行原始数据处理和锆石U-Pb谐和图绘制.普通铅校正使用204Pb进行.年龄误差为1σ绝对误差,同位素比值的误差为1σ相对误差;206Pb/238U年龄加权平均值为95%的置信度误差.
全岩主微量分析测试工作在国家地质实验测试中心进行.主量元素分析测试使用X射线荧光光谱议(XRF-1500),其分析精度高于5%.微量以及稀土元素测试分析使用等离子质谱仪(PE300D),分析结果以10-6为单位.
3. 分析结果
3.1 SHRIMP锆石U-Pb年龄
样品20181206-1锆石虽然形态各不相同,但是阴极发光图像下显示出典型的岩浆生长震荡环带和韵律结构.排除裂隙发育和较多包裹体的锆石颗粒,其余的锆石晶型完好,颜色较浅,呈透明、半透明钝圆形或者短柱状,锆石粒度变化范围为100~200 μm.所有锆石分析点均位于明显的岩浆环带位置.对15颗锆石进行U-Pb定年,获得15组数据和对应的锆石年龄,锆石的Th/U比值均大于0.4,显示岩浆锆石成因.剔除掉2个不协和年龄(6.1、15.1),其余13个数据点全部位于谐和线或附近,且年龄值比较集中(图 5),它们的206Pb/238U年龄加权平均值为1 011.9±8.9 Ma,MSWD=1.5,该年龄代表了天宝山组流纹岩的形成时代.
样品20181207-7锆石虽然具有不同的形态,但是在阴极发光图像下可以看到几乎都显示出典型的岩浆生长震荡环带与韵律结构.去除有裂隙且含有较多包裹体的锆石颗粒,其他锆石晶型完好,颜色较浅,呈透明或半透明钝圆形或者不规则状,锆石粒度变化范围为50~150 μm.所有锆石的分析点均位于明显的岩浆环带位置.对16颗锆石进行U–Pb定年,获得16组数据和对应的锆石年龄,锆石的Th/U比值均大于0.4,显示岩浆锆石成因特点.测点6.1打到了锆石增生边上,导致年龄偏小.剔除测点6.1,其余15数据点均位于谐和线上或附近,且年龄值比较集中(图 5),它们的206Pb/238U年龄加权平均值为910.6±4.7 Ma,MSWD=0.91.该年龄代表基性侵入岩的形成时代.
3.2 元素地球化学
辉长岩与流纹岩经历了不同的变质作用及蚀变作用,其烧失量值为1.73~3.58,大离子亲石元素等活泼元素在蚀变及变质过程中易发生迁移,因此需要选用不活泼的元素来进行判别.在低级-高级变质作用以及热液蚀变过程中,Zr元素是最稳定的元素,是很好的地球化学判别指示剂(Zhou et al., 2006a).不活泼元素主要包括Nb、Ta、Ti和Hf等高场强元素和Y、Th和U等稀土元素,活泼元素主要包括Rb、Na、K、Ca、Sr和Ba等.不活泼元素与Zr具有很好的线性相关性,而活泼元素则与Zr不存在线性相关性(图略).因此,本文采用不活泼元素来进行岩石分类以及成因分析.原始数据去除挥发分后对主量元素重新按100%进行换算.
辉长岩SiO2含量为46%~50%,Al2O3含量为16.3%~18.6%,MgO含量为4.80%~7.52%,CaO为7.06%~9.45%,Na2O含量为1.95%~3.82%,在Zr/TiO2-Nb/Y图解中,样品落入亚碱性玄武岩范围内(图 6),在FeOt/MgO-SiO2图解中,所有样品落入拉斑系列范围内(图 6),显示出亚碱性拉斑玄武岩特征.辉长岩样品稀土总量为54×10-6~98×10-6,(附表2),轻、重稀土元素分馏不明显,LREE/HREE=2.34~5.03,在球粒陨石标准化图解中呈平缓的右倾模式,(La/Yb)N=1.46~4.72,Eu异常不明显,δEu=0.81~1.31,整体与E–MORB配分模式相类似(图 7).在原始地幔微量元素蛛网图中(图 7),辉长岩相对富集大离子亲石元素,具有明显的Nb、Ta及Ti负异常,与岛弧玄武岩类似(Tamura et al., 2014).
图 7 天宝山组流纹岩及盐边群辉长岩稀土元素球粒陨石标准化图和微量元素原始地幔标准化图(Sun and McDonough, 1989)Fig. 7. Chondrite-normalized REE diagrams and primitive mantle-normalized incompatible trace element multi-element plots for the Tianbaoshan rhyolites (a and b) and the gabbros (c and d)(modified from Sun and McDonough, 1989)天宝山组流纹岩SiO2含量含量较高,为66% ~79%,Al2O3含量为10.6%~15.6%,K2O含量较高,为2.95%~5.66%,MgO含量较低,为0.48%~0.85%,CaO为0.21%~0.29%,Na2O含量为0.26%~2.66%(附表1),在Zr/TiO2-Nb/Y图解中,样品落入流纹岩范围内,在SiO2-FeOt/(FeOt+MgO)图解中,样品全部落入铁质系列范围内(图 6),与典型A型花岗岩相似(Whalen et al., 1987; Eby, 1992).在SiO2-K2O图解中,样品绝大部分落入高钾范围内(图 6).因此,天宝山组流纹岩属于高钾铁质流纹岩系列.流纹岩样品具有较高的稀土含量,总量为292×10-6~401×10-6,轻、重稀土元素中等分馏,LREE/HREE=3.03~6.46,稀土配分曲线呈右倾型式(图 7),(La/Yb)N=1.77~6.74(附表2),Eu具有明显负异常,δEu=0.43~0.56.在原始地幔微量元素蛛网图中(图 7),相比高场强元素,流纹岩更富集大离子亲石元素,具有明显的Nb、Ta、Sr、P和Ti负异常,10 000×Ga/Al=2.6~4.6.
4. 讨论
4.1 年代学意义
会理群上部天宝山组因发育大量中–酸性火山岩而被广泛关注,耿元生等(2017)在天宝山组流纹岩中分别获得1 028±13 Ma、1 018±11 Ma的SHRIMP锆石U-Pb年龄.尹福光等(2012)获得天宝山组英安岩SHRIMP锆石U-Pb年龄为1 036±12 Ma,Zhu et al.(2016)对会理洪川桥地区天宝山组酸性火山岩和基性岩脉进行定年,分别获得1 023.0±6.7 Ma、1 021.0±6.4 Ma、1 025±13 Ma和1 023.0±6.7 Ma的SHRIMP锆石U-Pb年龄. Chen et al.(2018)获得天宝山组英安岩Cameca SIMS锆石U-Pb年龄为1 032±27 Ma和1 063±41 Ma.本文获得的1 011.9±8.9 Ma的天宝山组上部层位的流纹岩年龄在误差范围内与上述年龄一致,说明会理群顶界年龄应在1 000 Ma左右.区内与之相当层位的火山岩事件有登相营群则姑组中-酸性火山岩,时代为1 030±19 Ma(耿元生等, 2008);滇中昆阳群黑山头组富良棚段凝灰岩SHRIMP锆石U-Pb年龄为1 031 Ma(张传恒, 2007);元谋地区苴林群普登组变质玄武岩时代为1 050 Ma(Chen et al., 2014),与天宝山组均为同一期火山活动的产物.会理群下部力马河组由于缺乏合适的测年对象而缺少直接的定年结果,Greentree(2006)在滇中地区与会理群层位相当的昆阳群下部老吴山组凝灰岩中获得1 142±16 Ma的SHRIMP锆石U-Pb年龄,会理群下伏通安组三段火山岩锆石U-Pb年龄为1 270±95 Ma(尹福光等, 2012).综合以上数据,笔者认为会理群形成时代为1 200~1 000 Ma.
前人对盐边群地层时代的限定大多基于碎屑锆石和侵入其中的岩体.盐边群下部中-基性火山岩的直接定年年龄为841±10 Ma(耿元生等, 2008)和825±10 Ma(张传恒, 2007). Li et al.(2006)获得侵入盐边群的2期岩浆岩年龄,将其中较老的一期(920~900 Ma)作为盐边群的下限年龄. Zhou et al. (2006a)于盐边群碎屑锆石获得840 Ma的最大沉积年龄. Sun et al. (2008b)获得盐边群碎屑岩最大沉积年龄为870 Ma.对比已经报道的年龄,盐边群的地层时代仍然没有得到较好的限制.本文获得侵入小坪组的辉长岩的SHRIMP锆石U-Pb年龄为910 Ma,说明盐边群小坪组及之下的地层形成时代应不晚于910 Ma.根据年代学数据和地层接触关系,盐边群的层位位于天宝山组之上,但是传统年代学证据显示两者之间缺少近150~200 Ma的地质记录.本文获得的910 Ma的侵入盐边群的辉长岩年龄以及1 011 Ma的天宝山组流纹岩年龄将盐边群底界年龄限制在1 000~910 Ma之间,说明盐边群是紧随天宝山组之后的沉积序列.值得注意的是,野外并未观察到辉长岩与盐边群顶部乍古组的侵入关系,根据本文数据,乍古组的地层时代可能晚于910 Ma,Sun et al. (2008a)获得侵入于盐边群的关刀山岩体的SHRIMP锆石U–Pb年龄为858±7 Ma,说明盐边群顶界年龄应不晚于860 Ma.综上所述,盐边群形成时代为 > 910~860 Ma.
扬子陆块西南缘康滇地区是我国南方元古代基底地出露最集中的地区,也是研究构造演化的重要窗口,但是由于基底地层呈断块出露,加之缺少精确的年代学数据,导致本区元古代基底地层划分对比长期存在分歧.本文侵入盐边群基性岩脉和天宝山组流纹岩的测年结果说明扬子西南缘中-新元古代发育多期次不同性质的构造热事件,也为准确地标定会理群和盐边群的地层时代和层序提供了新的年代学证据.
图 8 天宝山组流纹岩地球化学岩石判别图解FG.分异的I、S和M型花岗岩;OGT.未分异的I、S和M型花岗岩;据Whalen et al.(1987)Fig. 8. Geochemical discrimination diagrams for the felsic igneous rocks in the Tianbaoshan Formation4.2 岩石成因及构造背景
4.2.1 流纹岩
天宝山组流纹岩富集大离子亲石元素,具有明显的Nb、Ta、Sr、P和Ti负异常,同时具有较高的Na2O+K2O、FeOt/(FeOt+MgO)和Ga/Al值,与A型花岗岩地球化学组成类似,在Zr-10 000×Ga/Al和Nb-10 000×Ga/Al判别图解中,所有样品都落入了A型花岗岩范围内;在FeOt/MgO和10 000×Ga/Al–Zr+Nb+Y+Ce判别图解中,所有样品都落入A型花岗岩范围内.值得注意的是,高分异I型花岗岩与A型花岗岩具有相似的地球化学特征(Landenberger and Collins, 1996),因此我们需要排除高分异I型花岗岩的可能性.相比高分异I型花岗岩,A型花岗岩具有高的Na2O+K2O, Fe/Mg, 10 000×Ga/Al值以及较低的CaO和Sr含量(Whalen et al., 1987; Eby, 1992),因此天宝山组流纹岩属于典型的A型花岗岩,不同于I型花岗岩.
关于A型花岗岩的成因存在不同的观点,主要有以下几个观点:(1)幔源玄武质岩浆的直接分离结晶; (2)幔源基性岩浆和壳源岩浆的混合作用; (3)地壳物质的部分熔融(Whalen et al., 1987).笔者认为,天宝山组流纹岩形成于幔源玄武质岩浆分离结晶或者壳幔混染的可能性较小,主要证据有3点:(1)天宝山组流纹岩SiO2含量普遍偏高,同时MgO含量又相对偏低(< 0.9%),因此来自部分熔融的地幔熔体的直接分离结晶可能性较小(Takagi et al., 1999);(2)天宝山组流纹岩与区域上同时期的基性岩在主微量元素组成上没有相关性(图略),来自相同物源的可能性较小;(3)扬子西南缘中元古代末期岩浆岩主要以中酸性岩为主,而同时期的基性岩所占比例较少.天宝山组流纹岩最有可能的成因是先期古老地壳物质的部分熔融.另外,流纹岩MgO含量较低,这可能与富Mg矿物的分离结晶有关;Eu和Sr的负异常与斜长石的分离结晶作用有关;Ti的负异常说明存在钛铁矿的分离结晶.因此,成岩作用后期的分离结晶作用在天宝山组流纹岩形成过程中也起到重要作用.
A型花岗岩主要在伸展环境下产出(Eby, 1992),在Nb-Y和Rb-(Y+Nb)构造判别图解中(Pearce et al., 1996),天宝山组样品全部落在WPG(板内花岗岩)范围(图 10).天宝山组流纹岩与研究区内同时期的基性岩具有双峰式火山岩特征(Chen et al., 2014; Zhu et al., 2016),因此天宝山组流纹岩应形成于板内伸展环境.与伸展相关的构造背景包括陆块碰撞初期的撞击裂谷、后造山伸展和大陆裂谷等(Whalen et al., 1987; Eby, 1992).部分学者根据扬子陆块西南缘碎屑岩年龄谱系中出现的1 400 Ma的碎屑锆石年龄峰值提出扬子与华夏陆块在1 000 Ma时期已经发生碰撞拼贴(Greentree et al., 2006),扬子西南缘中元古代晚期岩浆岩形成于板块碰撞背景下的撞击裂谷(Zhu et al., 2016);Wang et al.(2019b)于会理群天宝山组识别出A1和A2两类同时代的酸性岩,认为其形成于活动大陆陆缘的弧后伸展环境.撞击裂谷及后造山伸展背景下产生的基性岩的微量元素具有明显的Nb-Ta负异常特征(Saunders and Tarney, 1984; Chen et al., 2014),根据前人研究,研究区内与天宝山组流纹岩同时代的基性岩不存在明显的Nb-Ta负异常(Chen et al., 2014; Zhu et al., 2016).另外,Cawood et al. (2018)根据古生物化石研究结果提出扬子西南缘中元古代晚期碎屑岩锆石中1 400 Ma的年龄峰值不一定来自华夏陆块,它们更有可能来自印度和澳大利亚陆块. Jiang et al. (2014)通过对比扬子陆块和印度陆块北西缘新元古代岩石组合,认为扬子陆块在新元古代时期更靠近印度陆块北西缘,而不是华夏陆块.形成于弧后盆地的基性岩具有岛弧拉斑玄武岩和N-MORB的“双模式”组合特征,而研究区内同时期的基性岩却不具有岛弧岩浆岩和N-MORB的特征.通过对比分析前人研究结果,天宝山组流纹岩应形成于板内裂谷环境.
4.2.2 辉长岩
侵入盐边群的辉长岩具有明显的Nb-Ta负异常,无明显Zr-Hf异常,说明辉长岩没有经历地壳物质的混染,这是因为大陆地壳具有亏损Nb、Ta元素,富集Zr、Hf元素的特点(Sun and McDonough, 1989),受到地壳混染的幔源岩浆在微量元素标准化蛛网图中势必会表现出明显的Nb-Ta负异常和Zr-Hf正异常.在微量元素Ta/La-La/Sm和Th/Ta-Nb/La图解中,辉长岩表现出分离结晶(FC)作用趋势,而与分离结晶混染作用(AFC)不一致(图 9).幔源岩浆具有高Ni(> 400×10-6)、高Cr(> 1 000×10-6)和高Mg#(73~81)的特征(Sharma, 1997).侵入盐边群的辉长岩MgO、Ni和Cr含量较低,说明存在橄榄石和辉石的分离结晶.样品Eu负异常不明显,说明斜长石的分离结晶作用较弱(图 6).原始地幔微量元素蛛网图中,辉长岩具有明显的Nb–Ta以及Ti负异常,与岛弧玄武岩微量元素配分模式相一致.较低的Nb(1.15~3.65)含量和Nb/La(0.1~0.3)比值与典型的岛弧岩浆岩一致(Nb < 4,Nb/La < 0.9)(Tamura et al., 2014).另外,辉长岩具有高Ba/Th(165~1 326), U/Th(0.27~0.54)以及Ba/Nb(111~456)比值,富集大离子亲石元素和轻稀土,亏损高场强元素,进一步说明辉长岩来源于俯冲带之上富水的岩石圈地幔楔,并且遭受到来自俯冲板片的流体/熔体的交代变质(Sun and McDonough, 1989).一些不活泼的微量元素在构造背景判别中可以起到很好的指示作用,在Hf/3-Th-Ta判别图解中(图 10),辉长岩样品全部落入火山岛弧玄武岩范围.在Nb×2-Zr/4-Y判别图解中(图 10),所有样品点都落在了火山岛弧范围.综上,笔者认为侵入盐边群的辉长岩是与板块俯冲相关的岛弧岩浆岩.
图 10 盐边群辉长岩(a) Th-Ta-Hf/3和(b) Zr-Ti构造判别图解和天宝山组流纹岩(c)Nb-Y和(d)Rb-(Y+Nb)构造判别图解图a中:A.正常洋中脊玄武岩; B.富集型洋中脊玄武岩与板内拉斑玄武岩; C.板内碱性玄武岩; D.岛弧拉斑玄武岩; 据Meschede(1986);图b中:AI.板内碱性玄武岩; AII.板内碱性玄武岩与拉斑玄武岩; B.富集型洋中脊玄武岩; C.板内玄武岩和岛弧玄武岩; D.富集型洋中脊玄武岩与岛弧玄武岩; C+D.岛弧玄武岩; a, b.据Wood(1980);c, d.据Pearce(1996)Fig. 10. Discrimination diagrams of (a) Th-Ta-Hf/3 plot and (b) Zr-Ti plot for the Yanbian gabbros and (c) Nb-Y plot and (d) Rb–Y+Nb plot for the Tianbaoshan rhyolite盐边群碎屑岩为深海、半深海沉积的浊积岩及硅质岩;火山岩为海相基性、中基性火山岩,结合沉积相和古地理构造格局,盐边群应属于弧后盆地沉积序列. Zhou et al.(2006a)获得攀枝花地区806 Ma和812 Ma的基性侵入岩,认为其形成于陆缘岛弧环境;Sun et al. (2008a)获得侵入盐边群的关刀山闪长岩体SHRIMP锆石U-Pb年龄为860 Ma,具有岛弧火山岩特征,形成于俯冲带环境. Sun et al. (2008b)提出扬子西南缘920~740 Ma期间存在长期的洋-陆俯冲.因此,笔者认为与盐边群配套的岩浆岛弧为扬子西南缘新元古代早-中期的岛弧岩浆岩. Crawford et al. (1981)提出,当洋壳开始俯冲时,岛弧岩浆岩开始发育,随着弧后伸展的持续,岛弧岩浆作用逐渐减弱.盐边群复理石沉积序列就是发育在此时的弧后盆地内.随后与俯冲相关的岩浆作用再次活动,在弧后盆地形成一系列岛弧火山岩及侵入岩.
4.3 构造意义
对扬子西南缘中元古代晚期-新元古代早期岩浆岩构造背景的制约将对扬子陆块在格林维尔期的构造演化认识提供重要证据.扬子与华夏陆块的碰撞造山过程长期以来一直存在争议.部分学者认为扬子与华夏陆块于1 000 Ma左右开始由西向东逐渐拼贴,于900 Ma左右完成最终拼合,属于格林维尔造山运动的一部分(Greentree et al., 2006; Li et al., 2009; Zhu et al., 2016).近几年越来越多的华南新元古代岩浆岩的年代学及地球化学数据证明扬子与华夏陆块的拼贴时限为820 Ma,甚至更晚,华南地区不存在格林威尔造山运动,新元古代(1 000~820 Ma或1 000~760 Ma)岩浆活动形成于持续的板块俯冲背景下,非地幔柱成因(Zhou et al., 2006a, 2006b; Wang et al., 2014; Zhao et al., 2018).
研究区内中元古代晚期岩浆活动频繁,如1 142 Ma的老吴山组碱性玄武岩(Greentree et al., 2006);苴林群1 050 Ma的变质玄武岩(Chen et al., 2014);会理群和苴林地区1 050 Ma的A型酸性岩(Chen et al., 2018)以及本文获得的1 019 Ma的天宝山组流纹岩.地球化学数据显示上述岩浆岩均形成于板内伸展背景下.通过对比分析前人研究结果,笔者认为扬子西南缘在中元古代晚期(1 100~1 000 Ma)不存在板块俯冲及碰撞造山事件,而是持续的板内裂谷活动,扬子西南缘中元古代晚期不存在格林维尔造山运动.
扬子西南缘新元古代岩浆岩成因及构造背景是近年来研究的热点.部分学者认为900~740 Ma的岩浆岩是地幔柱活动的产物,标志着华南格林维尔期造山运动结束之后的Rodinia超大陆的裂解(Li et al., 2006),对于上述岩浆岩所表现出来的岛弧属性,Li et al.(2006)认为其继承了先期受板片流体/熔体交代的地幔楔的地球化学特征,并不代表其形成时的构造环境.本文获得的1 011 Ma的天宝山组A型酸性岩及研究区内同时期的基性岩形成于板内伸展环境(Chen et al., 2014, 2018),否认扬子西南缘中元古代晚期-新元古代早期存在板块碰撞拼贴作用.另外,扬子西南缘发育的新元古代早期岩浆岩,如860±4 Ma侵入盐边群的关刀山岩体(Sun et al., 2008a),806 Ma的侵入盐边群的高家村岩体和812±3 Ma的冷水箐岩体(Zhou et al., 2006a),840~835 Ma的米易过铝质花岗岩(Zhu et al., 2019b)等,均形成于板块俯冲背景下. Zhao et al.(2017)在扬子西缘石棉SSZ型蛇绿岩中获得800 Ma的成岩年龄. Hu et al.(2017)获得侵入石棉蛇绿岩的辉长岩SHRIMP锆石U-Pb年龄为937 Ma. Du et al. (2014)提出扬子西南缘新元古代存在一个长期的岛弧岩浆活动带(Zhou et al., 2006b; Sun et al., 2009; Zhao et al., 2018),洋-陆俯冲开始时间为860 Ma.本文获得的910 Ma的辉长岩具有岛弧特征,形成于俯冲背景下.结合前人研究,笔者认为扬子西南缘新元古代早-中期存在持续的洋-陆俯冲作用,不存在碰撞造山运动,1 000~910 Ma期间板块运动机制由伸展转为挤压.
任光明等(2017)于会理菜子园蛇绿混杂岩中的辉长岩获得1 375 Ma的SHRIMP锆石U-Pb的年龄,证实该套蛇绿岩带形成于中元古代.扬子西南缘在中元古代中期(~1 400 Ma)存在古大洋,上扬子陆块和滇中地区并未直接相连.需要注意的是,传统上认为江南造山带西段经过雪峰山后开始转向南南西,经湘黔桂交界区进入广西,直达北部湾地区,该界限被认为是华夏与扬子陆块的西部分界线(Zhu et al., 2019).那么,康滇地轴南部也就是滇中地区应是独立于上扬子陆块和华夏陆块的微陆块(图 11).该陆块北侧、南侧分别与扬子、华夏陆块在不同时期发生俯冲以及碰撞拼贴.前人研究表明扬子西缘中元古代晚期-新元古代早期的基底地层普遍发育近东西向构造(李献华等, 2008),因此洋壳应向北俯冲于扬子陆块之下.结合前人研究,本文提出新的扬子陆块西南缘中元古代晚期-新元古代早期的构造演化模式:(1)1 375~1 000 Ma,扬子西南缘发生持续的板内伸展活动,会理菜子园地区已拉伸至洋壳,在被动陆缘南北两侧,分别沉积了中元古代晚期的昆阳群和会理群;(2)1 000~910 Ma,板块运动机制由被动陆缘伸展环境转变为活动陆缘挤压环境,洋壳向北俯冲于扬子陆块之下,形成一系列岛弧岩浆岩,弧后地区发生伸展,沉积了以新元古代盐边群为代表的弧后盆地沉积(图 12).
5. 结论
(1) 扬子陆块西南缘天宝山组上部流纹岩SHRIMP锆石U-Pb年龄为1 011 Ma,侵入盐边群的辉长岩SHRIMP锆石U-Pb年龄为910 Ma,为扬子西南缘中-新元古代地层划分对比提供了新的年代学证据.
(2) 天宝山组流纹岩来自古老地壳物质部分熔融,具有A型花岗岩特征,形成于大陆裂谷环境.盐边群辉长岩来自被俯冲板片流体/熔体交代的岩石圈地幔,具有岛弧特征,形成于板块俯冲背景.
(3) 与大陆裂谷相关的天宝山组流纹岩和与大陆岛弧相关的辉长岩记录了扬子西南缘1 000~900 Ma之间板块运动机制发生了转变,由伸展环境转变为汇聚环境.
致谢:本文由中国地质调查局地质调查项目《江南造山带区域地质调查片区总结与服务产品开发》(No. 121201111120117)以及《全国陆域及海区地质图件更新与共享》(No.DD20190370)项目所支持.北京离子探针中心杨淳、江南女士,谢士稳博士在样品接收、制样、仪器调试监控和数据处理方面提供了帮助,在此致以衷心的感谢.
附表数据见本刊官网(www.earth-science.net).
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图 1 (a) 华南板块构造格架简图;(b)扬子西南缘川滇地区元古代地层分布图(据耿元生等, 2017修改)
Fig. 1. (a) Simplified tectonic framework of the South China block; (b)geological map of the distribution of Proterozoic strata in Yunnan–Sichuan provinces(modified from Geng et al., 2017)
图 4 (a) 天宝山组流纹岩; (b)侵入盐边群的辉长岩; (c)会理群天宝山组流纹岩镜下特征; (d)侵入盐边群辉长岩镜下特征
Qz.石英; Pl.斜长石; Ch.绿泥石; Hb.角闪石; Bt.黑云母
Fig. 4. (a)Field photo of the rhyolites in the Huili Group; (b) field photos of gabbros the in the Yanbian Group; (c)microscopic photo of the rhyolites in the Huili Group; (d) microscopic photo of the gabbros in the Yanbian Group
图 7 天宝山组流纹岩及盐边群辉长岩稀土元素球粒陨石标准化图和微量元素原始地幔标准化图(Sun and McDonough, 1989)
Fig. 7. Chondrite-normalized REE diagrams and primitive mantle-normalized incompatible trace element multi-element plots for the Tianbaoshan rhyolites (a and b) and the gabbros (c and d)(modified from Sun and McDonough, 1989)
图 8 天宝山组流纹岩地球化学岩石判别图解
FG.分异的I、S和M型花岗岩;OGT.未分异的I、S和M型花岗岩;据Whalen et al.(1987)
Fig. 8. Geochemical discrimination diagrams for the felsic igneous rocks in the Tianbaoshan Formation
图 10 盐边群辉长岩(a) Th-Ta-Hf/3和(b) Zr-Ti构造判别图解和天宝山组流纹岩(c)Nb-Y和(d)Rb-(Y+Nb)构造判别图解
图a中:A.正常洋中脊玄武岩; B.富集型洋中脊玄武岩与板内拉斑玄武岩; C.板内碱性玄武岩; D.岛弧拉斑玄武岩; 据Meschede(1986);图b中:AI.板内碱性玄武岩; AII.板内碱性玄武岩与拉斑玄武岩; B.富集型洋中脊玄武岩; C.板内玄武岩和岛弧玄武岩; D.富集型洋中脊玄武岩与岛弧玄武岩; C+D.岛弧玄武岩; a, b.据Wood(1980);c, d.据Pearce(1996)
Fig. 10. Discrimination diagrams of (a) Th-Ta-Hf/3 plot and (b) Zr-Ti plot for the Yanbian gabbros and (c) Nb-Y plot and (d) Rb–Y+Nb plot for the Tianbaoshan rhyolite
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